1. Field of the Invention
The present invention relates to a high voltage inverter, and particularly, to a multilevel inverter using a cascade configuration with a device for equally controlling the power factors of a plurality of power cells, and to a control method thereof.
2. Background of the Invention
In general, a multilevel inverter using a cascade configuration is a multilevel topology of a high voltage and large capacity inverter, in which several is single phase inverters (hereinafter, referred to as power cells or cells) are connected in series for each phase of a three-phase current and accordingly a high voltage can be obtained by using low voltage power semiconductor switches within the power cells.
Thus, a multilevel inverter using a cascade configuration can obtain a high voltage using a semiconductor switch by serially connecting multiple single-phase power cells to each AC (Alternating Current) phase.
Also, a multilevel inverter using a cascade configuration has a feature of a pulse width modulation/phase shift wherein a phase difference is sequentially generated between power cells which are serially connected to one another. Accordingly, the multilevel inverter can have a low rate of output voltage change (dv/dt). In addition, the multilevel inverter using the cascade configuration can obtain reduced total harmonic distortion due to an output voltage with multi levels, namely, many steps.
Further, the multilevel inverter using the cascade configuration rarely incurs a voltage reflection. Accordingly, in spite of a long distance between the multilevel inverter using the cascade configuration and a motor, there is no need for a separate device to prevent the voltage reflection phenomenon.
However, in the multilevel inverter using the cascade configuration, each cell includes an AC-to-DC converter for an AC (Alternating Current) input, composed of a rectifier circuit and a smoothing circuit, independent of each other. Therefore, the multilevel inverter using the cascade configuration requires a separately insulated power source for each power cell, thereby increasing the required provision of secondary windings. Also, the multilevel inverter using the cascade configuration generates a phase difference due to the use of a phase shift transformer having a phase delay for each output, and also changes the phase for each of, for example, 8 power cells with respect to each of the three U, V, W phases due to an inductance component of a load or the like, which causes the power factors, namely, cosine values for the respective phases to be different from one another.
If such power factor difference is ignored, the level of the voltage regenerated in each power cell changes upon controlling a motor to be decelerated. Specifically, for a small load, if the value of the power factor difference is not compensated for, the regeneration voltage is converged in a particular (power) cell, which may cause the corresponding cell to suffer trouble or be burnt out, resulting in that driving of the multilevel inverter using the cascade configuration becomes impossible.